Bis-(2,5-polythio-1,3,4-thiadiazoles), rubbers containing such compounds, and a method of preparation of bis-(2,5-polythio-1,3,4-thiadiazoles)

ABSTRACT

This invention relates to bis-(2,5-polythio-1,3,4-thiadiazoles), a method for their preparation and their use in rubber compounds. Use of these compounds provide for excellent vulcanization of rubbers and result in an improved final rubber vulcanizate possessing good physical properties.

This is a divisional of application Ser. No. 08/189,332, filed on Jan.31, 1994, presently pending, which is a divisional of application Ser.No. 07/935,324, filed on Aug. 26, 1992, now U.S. Pat. No. 5, 310,921.

BACKGROUND OF THE INVENTION

This invention relates to bis-(2,5-polythio-1,3,4-thiadiazoles) andtheir use in the vulcanization of natural rubber, a rubber derived froma diene monomer and mixtures thereof.

In the manufacture of rubber articles, crude or raw rubber is compoundedwith various ingredients among which are sulfur and accelerators. Theprimary function of an accelerator or accelerator system is to increasethe rate of the vulcanization process while allowing sufficient time tomix the accelerators into the rubber at an elevated temperature beforevulcanization commences. Many accelerator combinations have been used inthe rubber industry. Unfortunately, many of the known accelerators, suchas morpholine containing compounds, dimethyl amine containing compoundsand dithiocarbamate compounds yield volatile nitrosoamines upon use. Theuse of compounds which yield volatile nitrosoamines have beensignificantly restricted in a number of countries and the need to find asuitable replacement is imminent.

SUMMARY OF THE INVENTION

The present invention relates to bis-(2,5-polythio-1,3,4-thiadiazoles)and their use in rubber stocks.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed bis-(2,5-polythio-1,3,4-thiadiazoles) consisting ofthe formula: ##STR1## wherein the sum of x and y is from 1 to 16 and xand y are independently selected from 0 and integers of from 1 to 8.

There is also disclosed a process for accelerating the cure rate of arubber stock comprising admixing (1) a rubber selected from the groupconsisting of natural rubber, synthetic rubbers derived from a dienemonomer and mixtures thereof with (2) abis-(2,5-polythio-1,3,4-thiadiazole) consisting of the formula: ##STR2##wherein the sum of x and y is from 1 to 16 and x and y are independentlyselected from 0 and integers of from 1 to 8.

There is also disclosed a rubber stock which comprises (1) a rubberselected from the group consisting of natural rubber, synthetic rubbersderived from a diene monomer and mixtures thereof, and (2) abis-(2,5-polythio-1,3,4-thiadiazole) consisting of the formula: ##STR3##wherein the sum of x and y is from 1 to 16 and x and y are independentlyselected from 0 and integers of from 1 to 8.

The present invention also relates to a process for preparation of theabove bis-(2,5-polythio-1,3,4-thiadiazoles) comprising reacting2,5-dimercapto-1,3,4-thiadiazole and elemental sulfur in water withhydrogen peroxide at a temperature ranging from 20° C. to 100° C.wherein the molar ratio of 2,5-dimercapto-1,3,4-thiadiazole to hydrogenperoxide is 1:1 and the molar ratio of 2,5-dimercapto-1,3,4-thiadiazoleto elemental sulfur ranges from 16:1 to 1:1.

The reaction product of the above reaction may consist of a mixture ofbis-(2,5-polythio-1,3,4-thiadiazoles), all of which may vary inmolecular weight. For example, referring to the above structuralformula, there may be a mixture of the materials wherein x and y mayrange from 0 to 8 so long as the sum of x and y is at least 1 and up to16. Preferably, x and y are integers ranging from about 2 to 4. Themolecular weight of the reaction product will vary depending on theratio of reactants, temperature of the reaction and reaction time. Themolecular weight of the composition of the present invention may rangefrom about 328 to about 808. Preferably, the molecular weight of thereaction product ranges from about 424 to 552.

In accordance with the process of making thebis(2,5-polythio-1,3,4-thiadiazoles), the mole ratio of2,5-dimercapto-1,3,4-thiadiazole to sulfur may range from about 16:1 to1:1. Preferably, the mole ratio ranges from about 4:1 to 2:1.

The mole ratio of 2,5-dimercapto-1,3,4-thiadiazole to hydrogen peroxideshould range from about 1:1 to 1.5:1. Preferably, the mole ratio is from1:1 to 1.25:1.

In accordance with the present invention,2,5-dimercapto-1,3,4-thiadiazole and elemental sulfur in water arereacted with hydrogen peroxide. The term elemental sulfur is used hereinto describe the S₈ or rhombic form of sulfur.

The reaction may be conducted over wide temperatures. In general, thereaction may be conducted at a temperature of from about 20° C. to about100° C. Preferably, the condensation reaction is conducted at atemperature ranging from about 50° C. to about 95° C.

The reaction between the 2,5-dimercapto-1,3,4-thiadiazole, elementalsulfur and hydrogen peroxide may be conducted under a variety ofpressures, with atmospheric pressure being preferred. The reaction maybe conducted under an inert atmosphere or air. Preferably, theatmosphere is air.

The reaction is conducted for a time sufficient to produce the desiredproduct which upon heating and isolation will result in thebis-(2,5-polythio-1,3,4-thiadiazole). In general, the reaction time mayvary from about 1 hour to about 8 hours.

Upon completion of the reaction, the desiredbis-(2,5-polythio-1,3,4-thiadiazole) is isolated. The method ofisolation is conventional and well known to those skilled in the art andmay consist of permitting the reaction mixture to cool for subsequentuse or by simple vacuum filtration.

The bis-(2,5-polythio-1,3,4-thiadiazoles) may be used with a number ofrubber stocks. While the utility of these materials in rubber stocks mayvary depending on the amount used, the use as an accelerator has beenparticularly noted. In greater amounts, thebis-(2,5-polythio-1,3,4-thiadiazoles) may function as a sulfur donor.Examples of rubbers include substituted and unsubstituted, saturated andunsaturated, natural and synthetic polymers. The natural polymersinclude natural rubber in its various forms, e.g., pale crepe and smokedsheet, and balata and gutta percha. The synthetic polymers are derivedfrom a diene monomer and include those prepared from a single monomer(homopolymer) or a mixture of two or more copolymerizable monomers(copolymer) when the monomers are combined in the random distribution orblock form. The monomers may be substituted or unsubstituted and maypossess one or more double bonds, conjugated and nonconjugated dienesand monoolefins, including cyclic and acyclic monoolefins, especiallyvinyl and vinylidene monomers. Examples of conjugated dienes are1,3-butadiene, isoprene, chloroprene, 2-ethyl-1,3-butadiene,2,3-dimethyl-1,3-butadiene and piperylene. Examples of nonconjugateddienes are 1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene,dicyclopentadiene, 1,5-cyclooctadiene, and ethyldiene norbornene.Examples of acyclic monoolefins are ethylene, propylene, 1-butene,isobutylene, 1-pentene and 1-hexene. Examples of cyclic monoolefins arecyclopentene, cyclohexene, cycloheptene, cyclooctene and4-methyl-cyclooctene. Examples of vinyl monomers are styrene,acrylonitrile, acrylic acid, ethylacrylate, vinyl chloride,butylacrylate, methyl vinyl ether, vinyl acetate and vinyl pyridine.Examples of vinylidene monomers are alpha-methylstyrene, methacrylicacid, methyl methacrylate, itaconic acid, ethyl methacrylate, glycidylmethacrylate and vinylidene chloride. Representative examples of thesynthetic polymers used in the practice of this invention arepolychloroprene homopolymers of a conjugated 1,3-diene such as isopreneand butadiene, and in particular, polyisoprenes and polybutadieneshaving essentially all of their repeat units combined in acis-1,4-structure; and copolymers of a conjugated 1,3-diene such asisoprene and butadiene with up to 50% by weight of at least onecopolymerizable monomer, including ethylenically unsaturated monomerssuch as styrene or acrylonitrile; and butyl rubber, which is apolymerization product of a major proportion of a monoolefin and a minorproportion of a diolefin such as butadiene or isoprene. The rubberstocks which may be used with the bis-(2,5-polythio-1,3,4-thiadiazoles)preferably contain cis-1,4-polyisoprene (natural or synthetic),polybutadiene, polychloroprene and the copolymers of isoprene andbutadiene, copolymers of acrylonitrile and butadiene, copolymers ofacrylonitrile and isoprene, copolymers of styrene, butadiene andisoprene, copolymers of styrene and butadiene and blends thereof.

One advantage of the present invention is that thebis-(2,5-polythio-1,3,4-thiadiazoles) function as curative agents forrubber stocks and are believed to not form nitrosoamines. Thebis-(2,5-polythio-1,3,4-thiadiazoles) also function as sulfur donors inan activated nonblooming form. Therefore, thebis-(2,5-polythio-1,3,4-thiadiazoles) may be used in their own right asa rubber curative or as a co-agent in a cure system. While the amount ofbis-(2,5-polythio-1,3,4-thiadiazole) that is used in a rubber stock mayvary, it generally ranges from about 0.10 phr (parts by weight per 100parts by weight of rubber) to about 10 phr. Preferably, the amount whichis used ranges from about 0.25 phr to about 1.0 phr.

As known to one skilled in the art, in order to cure a rubber stock, oneneeds to have a sulfur vulcanizing agent. In accordance with oneembodiment, the bis-(2,5-polythio-1,3,4-thiadiazole) can function as thesulfur vulcanizing agent in whole or in part with conventional sulfurvulcanizing agents. Examples of suitable conventional sulfur vulcanizingagents which may be used include elemental sulfur (free sulfur) or asulfur donating vulcanizing agent, for example, an amine disulfide,polymeric polysulfide or sulfur olefin adducts. Preferably, theconventional sulfur vulcanizing agent is elemental sulfur. The amount ofsulfur vulcanizing agent will vary depending on the components of therubber stock and the particular type of sulfur vulcanizing agent that isused. Generally speaking the amount of sulfur vulcanizing agent rangesin an amount of from about 0.25 to about 10 phr. Preferably, the sulfurvulcanizing agent is present in an amount ranging from about 1.0 toabout 6.0 phr.

Conventional rubber additives may also be incorporated in the rubberstock. The additives commonly used in rubber stocks include fillers,plasticizers, processing oils, retarders, antiozonants, antioxidants andthe like. The total amount of filler that may be used may range fromabout 30 to about 80 phr, with a range of from about 45 to about 70 phrbeing preferred. Fillers include silicas, clays, calcium carbonate,calcium silicate, titanium dioxide and carbon black. HAF Black (N-330)and GPF-Black (N-660) are commonly used in rubber stocks intended foruse as wire coats or carcass ply coats. Preferably, at least a portionof the filler is carbon black. Plasticizers are conventionally used inamounts ranging from about 2 to about 50 phr with a range of about 5 toabout 30 phr being preferred. The amount of plasticizer used will dependupon the softening effect desired. Examples of suitable plasticizersinclude aromatic extract oils, petroleum softeners includingasphaltenes, saturated and unsaturated hydrocarbons and nitrogen bases,coal tar products, cumarone-indene resins and esters such asdibutylphthalate and tricresyl phosphate. Materials used in compoundingwhich function as an accelerator-activator include metal oxides such aszinc oxide, magnesium oxide and litharge which are used in conjunctionwith acidic materials such as fatty acid, for example, stearic acid,oleic acid, murastic acid, and the like. The amount of the metal oxidemay range from about 1 to about 10 phr with a range of from about 2 toabout 8 phr being preferred. The amount of fatty acid which may be usedmay range from about 0.25 phr to about 5.0 phr with a range of fromabout 0.5 phr to about 2 phr being preferred.

A class of compounding materials known as scorch retarders are commonlyused. Phthalic anhydride, salicyclic acid, sodium acetate andN-cyclohexyl thiophthalimide are known retarders. Retarders aregenerally used in an amount ranging from about 0.1 to 0.5 phr.

Preformed phenol-formaldehyde type resins which may be used in therubber stock and are generally present in an amount ranging from about1.0 to about 5.0 phr, with a range of from about 1.5 to about 3.5 phrbeing preferred.

Conventionally, antioxidants and sometimes antiozonants, hereinafterreferred to as antidegradants, are added to rubber stocks.Representative antidegradants include monophenols, bisphenols,thiobisphenols, polyphenols, hydroquinone derivatives, phosphites,thioesters, naphthyl amines, diphenyl-p-phenylenediamines,diphenylamines and other diaryl amine derivatives,para-phenylenediamines, quinolines and mixtures thereof. Specificexamples of such antidegradants are disclosed in The Vanderbilt RubberHandbook (1990), pages 282-286. Antidegradants are generally used inamounts from about 0.25 to about 5.0 phr with a range of from about 1.0to about 3.0 phr being preferred.

The present invention may be better understood by reference to thefollowing examples in which the parts or percentages are by weightunless otherwise indicated.

EXAMPLE 1 Preparation of Bis(2,5-tetrathio-1,3,4-thiadiazole)

In a 1000 milliliter reaction flask, a mixture of 30 grams (0.20 mole)of 2,5-dimercapto-1,3,4-thiadiazole and 12.8 grams (0.05 mole) ofelemental sulfur was prepared in 250 ml of water. The slurry was stirredat high speed and a pale yellow suspension resulted. To the slurry wasadded 29.4 grams (0.259 mole) of 30% hydrogen peroxide over a fiveminute period. During the addition of the peroxide, the temperatureincreased to 38° C. and the reaction slurry became thicker and brightorange. The slurry was stirred with heating until a temperature of 70°C. was obtained. Heating at 70° C. was continued for one hour. Duringthis heating period, the solids turned white. The heating was increasedto 100° C. and stirring continued for 5 hours. The solids were collectedby suction filtering and dried in a vacuum oven. 40.2 grams of productwere recovered (94.8% yield). The product exhibited shrinkage at 145° to155° C., followed by an amorphous state at 156° to 170° C. anddecomposed with gassing at 171° to 175° C.

EXAMPLE 2 Preparation of Bis(2,5-hexathio-1,3,4-thiadiazole)

In a 12 liter round bottom wide mouth flask equipped with a mechanicalmixer, a mixture of 900 grams (6.0 mole) of2,5-dimercapto-1,3,4-thiadiazole and 768 grams (3 mole) of elementalsulfur was prepared in 4 liters of water. The slurry was stirred at highspeed and a pale yellow suspension resulted. To the slurry was added 850grams (7.5 mole) of 30% hydrogen peroxide over a thirty minute periodmaintaining a reaction of 60° C. The reaction slurry became thicker anddark yellow. The slurry was stirred with heating until a temperature of95° C. was obtained. Heating at 95° C. was continued for two hours. Themixture was allowed to cool overnight and the solids were collected bysuction filtering. The solids were washed with water and then dried.1620.3 grams of product were recovered (97.8% yield). The productexhibited shrinkage at 145° to 150° C., followed by an amorphous stateat 150° to 155° C. and decomposed with gassing at 156° to 172° C.

EXAMPLE 3 Preparation of Bis(2,5-decathio-1,3,4-thiadiazole)

In a 12 liter round bottom wide mouth flask equipped with a mechanicalstirrer, a mixture of 900 grams (6 mole) of2,5-dimercapto-1,3,4-thiadiazole and 1536 grams (6 mole) of elementalsulfur was prepared in 4 liters of water. The slurry was stirred at highspeed and a pale yellow suspension resulted. To the slurry was added 850grams (7.5 mole) of 30% hydrogen peroxide over a thirty minute periodmaintaining a reaction temperature of 60° C. The reaction slurry becamethicker and dark yellow. The slurry was stirred with heating until atemperature of 95° C. was obtained. Heating at 95° C. was continued fortwo hours. The mixture was allowed to cool overnight and the solids werecollected by suction filtering. The solids were washed with water andthen dried. 2392.5 grams of product were recovered (98.7% yield). Theproduct exhibited shrinkage at 145° to 155° C., followed by an amorphousstate of 156° to 160° C. and decomposed with gassing at 161° to 175° C.

EXAMPLE 5 Physical Testing

Table I below shows the basic rubber stock that was used in thisexample. The rubber compound was prepared in a two-stage Banbury mix.All parts and percentages are by weight unless otherwise noted. The curedata as well as other physical data for each sample are listed in TableII or Table III.

                  TABLE I                                                         ______________________________________                                        Non-Productive                                                                Natural Rubber        100                                                     Carbon Black          50                                                      Wax                   1                                                       Antidegradant         3                                                       Zinc Oxide            3                                                       Fatty Acid            2                                                       Productive                                                                    Sulfur                1                                                       Accelerator           0.75                                                    Retarder              0.2                                                     Bis-(2,5-polythio-1,3,4-thiadiazole)                                                                Varied                                                  ______________________________________                                    

Cure properties were determined using a Monsanto oscillating discrheometer which was operated at a temperature of 150° C. and at afrequency of 1.7 hertz. A description of oscillating disc rheometers canbe found in the Vanderbilt Rubber Handbook edited by Robert O. Ohm(Norwalk, Conn., R. T. Vanderbilt Company, Inc., 1990), pages 554-557.The use of this cure meter and standardized values read from the curveare specified in ASTM D-2084. A typical cure curve obtained on anoscillating disc rheometer is shown on page 555 of the 1990 edition ofthe Vanderbilt Rubber Handbook.

In such an oscillating disc rheometer, compounded rubber samples aresubjected to an oscillating shearing action of constant amplitude. Thetorque of the oscillating disc embedded in the stock that is beingtested that is required to oscillate the rotor at the vulcanizationtemperature is measured. The values obtained using this cure test arevery significant since changes in the rubber or the compounding recipeare very readily detected. It is obvious that it is normallyadvantageous to have a fast cure rate.

The following tables report cure properties that were determined fromcure curves that were obtained for the rubber stocks that were prepared.These properties include a torque minimum (Min. Torque), a torquemaximum (Max. Torque), minutes to 25% of the torque increase (t25),minutes to 90% of the torque increase (t90) and difference between themaximum torque and minimum torque (delta torque).

Peel adhesion testing was done to determine the interfacial adhesionbetween various rubber formulations that were prepared. The interfacialadhesion was determined by pulling one compound away from another at aright angle to the untorn test specimen with the two ends being pulledapart at a 180 degree angle to each other using an Instron machine. Thearea of contact was determined from placement of a Mylar sheet betweenthe compounds during cure. A window in the Mylar allowed the twomaterials to come into contact with each other during testing.

The amount of heat buildup was measured with the Goodrich Flexometer asdescribed in ASTM Designation D623 (Method A). This test is described atpages 530-531 of The Vanderbilt Rubber Handbook, 13th Edition (1990).

The Mooney viscosity was measured at 100° C. in accordance with theequipment and method described at pages 565-566 of The Vanderbilt RubberHandbook, 13th Edition (1990).

The abrasion resistance was measured in accordance with German DIN 53516(DIN Abrasion).

Shore Hardness was determined in accordance with ASTM-1415.

Table II indicates the amounts of bis-(2,5-polythio-1,3,4-thiadiazole)that was used in each sample. The bis-(2,5-polythio-1,3,4-thiadiazole)was added to the rubber stocks in the productive stage.

                                      TABLE II                                    __________________________________________________________________________                       Control                                                                       Sample 1                                                                           Sample 2                                                                           Sample 3                                                                           Sample 4                                    __________________________________________________________________________    Bis(2,5-tetrathio-1,3,4-thiadiazole) (phr)                                                       0    1.0  2.0  3.0                                         Monsanto Fatigue (k · cycles)                                                           345  371  375  330                                         Rheometer                                                                     Torque Rate (slope)                                                                              3.55 3.46 3.18 3.15                                        Delta Torque (min.)                                                                              28.0 29.5 31.5 32.50                                       T 25 (min.)        8.5  3.44 3.10 2.95                                        T 90 (min.)        13.3 8.34 9.15 10.25                                       Stress Strain                                                                 300% Modulus (MPa) 12.96                                                                              13.95                                                                              13.70                                                                              14.00                                       Break Strength (MPa)                                                                             25.88                                                                              23.50                                                                              22.75                                                                              22.81                                       % Elongation       525  510  485  465                                         Hardness, RT       64   64   66   67                                          Zwick Rebound      61   63   63   65                                          Peel Adhesion (Newtons/mm)                                                                       50   40   35   33                                          Mooney Viscosity (%)                                                                             67   65   67   66                                          Goodrich Flex                                                                 Hardness (Shore A) 64   68   70   69                                          Compression Set (%)                                                                              2.1  2.0  2.0  1.8                                         Heat Build-up (°C.)                                                                       39   32   32   30                                          DIN Abrasion (mm.sup.3)                                                                          131  118  117  127                                         __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________               Control                                                                       Sample 1                                                                           Sample 2                                                                           Sample 3                                                                           Sample 4                                                                           Sample 5                                                                           Sample 6                                                                           Sample 7                             __________________________________________________________________________    Bis(2,5-hexathio-1,3,4-                                                                  0    1.0  2.0  3.0  0    0    0                                    thiadiazole) (phr)                                                            Bis(2,5-decathio-1,3,4-                                                                  0    0    0    0    1.0  2.0  3.0                                  thiadiazole) (phr)                                                            Monsanto Fatigue                                                                         440  361  331  209  374  201  193                                  (k · cycles)                                                         Rheometer                                                                     Torque Rate (slope)                                                                      3.25 3.75 3.84 3.41 4.00 4.25 3.70                                 Delta Torque (min.)                                                                      25.60                                                                              30.0 34.0 35.0 33.5 40.0 40.1                                 T 25 (min.)                                                                              7.35 3.7  3.2  3.0  4.0  3.25 3.1                                  Stress Strain                                                                 300% Modulus (MPa)                                                                       11.50                                                                              13.25                                                                              14.16                                                                              14.09                                                                              13.82                                                                              15.75                                                                              16.33                                Break Strength (MPa)                                                                     24.50                                                                              23.60                                                                              23.30                                                                              23.12                                                                              24.00                                                                              22.00                                                                              23.00                                % Elongation                                                                             550  500  475  480  500  425  430                                  Hardness, RT                                                                             62   65   67   67   66   69   70                                   Zwick Rebound                                                                            60   63   64   65   64   66   67                                   Mooney Viscosity (%)                                                                     70   68   68   69   69   60   63                                   Goodrich Flex                                                                 Hardness (Shore A)                                                                       65   69   70   70   69   72   73                                   Compression Set (%)                                                                      2.4  2.1  1.6  1.7  1.7  1.8  1.4                                  Heat Build-up (°C.)                                                               62   58   54   54   56   56   56                                   DIN Abrasion (mm.sup.3)                                                                  125  112  112  118  109  122  128                                  Peel Adhesion                                                                            52.5 40.5 29.5 28.5 39   25.25                                                                              18.75                                (Newtons/mm)                                                                  __________________________________________________________________________

As can be seen above, addition of bis(2,5-tetrathio-1,3,4-thiadiazole)to a rubber stock will result in an Increase in the compound state ofcure as measured by the increase in the Monsanto rheometer torqueincrease. This indicates an increase in the compound formulationcrosslink density. The compound tensile strength and modulus at 300%elongation is increased along with an increase in the compoundresilience or rebound characteristics. In addition heat build-up withinthe compound is reduced along with a directional improvement in abrasionresistance. These properties collectively will allow improvement in tireperformance especially with regard to tread wear and rolling resistance.Similar trends are evident with use ofbis(2,5-hexathio-1,3,4-thiadiazole) andbis(2,5-decathio-1,3,4-thiadiazole) such as: an increase in the state ofcure and crosslink density, an increase in compound 300% modulus, animprovement in hysteretic properties of the compound as measured byincrease in compound rebound, and decrease in compound compression set.In all cases use of these materials leads to improvement in abrasionresistance. In summary, when compounded for application in a rubberstock for use in a product such as a tire, the claimed materials willcontribute toward an improvement in the product's performance. Fortires, this will represent itself in improved tread wear, reducedrolling resistance performance, and reduction in tire servicetemperatures.

What is claimed is:
 1. A process for the preparation ofbis-(2,5-polythio-1,3,4-thiadiazole) comprising reacting2,5-dimercapto-1,3,4-thiadiazole and elemental sulfur in water withhydrogen peroxide at a temperature ranging from 20° C. to 100° C.wherein the molar ratio of 2,5-dimercapto-1,3,4-thiadiazole to hydrogenperoxide is 1:1 to 1.5:1 and the molar ratio of2,5-dimercapto-1,3,4-thiadiazole to elemental sulfur ranges from 16:1 to1:1.
 2. The process of claim 1 wherein the reaction is conducted at atemperature of from 50° C. to 95° C.
 3. The process of claim 1 whereinthe molar ratio of 2,5-dimercapto-1,3,4-thiadiazole to elemental sulfurranges from 4:1 to 2:1.
 4. The process of claim 1 wherein the elementalsulfur and 2,5-dimercapto-1,3,4-thiadiazole are initially introduced inthe form of a suspension in water, followed by the addition of hydrogenperoxide.
 5. The process of claim 1 wherein the molar ratio of2,5-dimercapto-1,3,4-thiadiazole to hydrogen peroxide ranges from 1:1 to1.25:1.